1. Field of the Invention
The field of this invention relates to devices which shift signals from one frequency to another. In particular, the invention provides a mixer which operates with frequencies in the microwave region and, more particularly, the mixer of the invention operates with a resonant loop to improve isolation between the local oscillator (LO) and RF ports in a resonant loop FET mixer.
2. Description of the Prior Art
FET based mixers have been investigated for many years with emphasis on active single or dual gate FET mixer designs. Drawbacks to the active FET mixer design include a high noise figure for dual gate designs and poor 1/f noise characteristics at low IF frequencies for both single and dual gate designs.
U.S. Pat. No. 4,949,398 to Maas is one example of a prior art mixer apparatus. The Maas '398 patent teaches a simple balanced-mixer circuit that uses the resistive channel of a GaAs Metal Semiconductor Field Effect Transistor (MESFET). The Maas '398 patent mixer employs dual MESFET's. The RF and IF signals are connected to the same FET terminals. While this approach is useful where the RF and LO frequencies are widely separated, it has now been found that, where the RF and LO frequencies are closely spaced, excellent isolation between the RF and local oscillator (LO) stages may be achieved with the use of a single FET mixer incorporating a resonant loop as taught by the present invention. In addition, the present invention, in contrast to the prior art, provides simplified IF filtering because the mixer of the present invention has the RF and IF signals connected to separate FET terminals.
Maas also describes a GaAs MESFET mixer in an article entitled "A GaAs MESFET Mixer with Very Low Intermodulation," IEEE Transactions on Microwave Theory and Techniques, Vol. MIT-35, No. 4, April 1987. Maas describes in some detail the small signal equivalent circuits of a MESFET in this article.
U.S. Pat. No. 4,308,473 to Carnes teaches a means for impressing a polyphase coded modulation on a transmitted signal and delayed replicas of such modulation on a bank of correlator/mixers. Each of the correlator/mixers include dual FETs as the active elements.
U.S. Pat. No. 4,896,374 to Waugh et al. provides a broadband monolithic balanced mixer apparatus. The '374 patent employs a common source FET with feedback for a signal splitter and a common gate FET for a combiner.
U.S. Pat. No. 4,160,213 to Carter provides a mixer injection voltage compensation apparatus. The '213 patent apparatus includes a varactor controlled LO and a FET arranged as a mixer.
U.S. Pat. No. 4,658,440 to Pavie et al. is another example of a mixer using dual FETs. The '440 patent provides a single balanced self-oscillating dual gate FET mixer which includes a dielectric resonator and two transmission lines inductively coupled to the resonator and to the second gates of the dual FETs.
U.S. Pat. No. 4,931,799 to Wen et al. provides a short-range radar transceiver employing a FET oscillator. The FET employed is varactor-tuned.
In contrast to the prior art it is an object of the present invention to provide a mixer using a single FET and resonant loop which exhibits excellent intermodulation performance at frequencies of about 35 GHz.